Docking with the ISS involves floating in a mostly gravity free environment, with no wind/air resistance, and moving via small controlled jet bursts. Landing a

Docking with the ISS involves floating in a mostly gravity free environment, with no wind/air resistance, and moving via small controlled jet bursts. Landing a rocket on a barge involves dealing with gravity, wind resistance, air currents and the crazily unstable centre-of-gravity of a large vertical tube balancing on some rockets.